Field of the Invention
The present invention relates to a method for determining the speed of a rotary wing aircraft with respect to the surrounding air, and an associated device.
Description of the Related Art
Anemometry systems for rotary wing aircraft, such as helicopters, are generally identical to those of airplanes and therefore based on measurements of total pressure, for example using Pitot tubes, and of static pressure.
Such systems are however poorly adapted to rotary wing aircraft, because they become inoperative at low speed, in particular when the aircraft is substantially in static flight, in lateral flight or in backward flight, which compels visual piloting.
Moreover, lack of knowledge of the local wind during takeoff from an area not equipped with an anemometer can result, for safety reasons, in limiting the payload carried.
Various methods have been proposed, since the 1970s, for trying to measure airspeed, i.e. the speed of the aircraft with respect to the surrounding air, when the conventional means are inoperative, whether by means of pressure measurement or of weathervanes placed in the vicinity of the rotor, or indirectly by combining different internal parameters.
These alternative methods remain complex and their use is essentially limited to flight tests or to high performance military carriers.
The document FR 2761162 discloses the measurement of the speed of a helicopter with respect to the surrounding air by a Doppler LiDAR device. This device is based on the measurement of the frequency shift, by the Doppler effect, between a laser beam transmitted into the atmosphere and the beam backscattered by the particles naturally present in the probed air volume. This Doppler frequency is directly proportional to the projection, along the sighting axis, of the relative speed of the carrier with respect to the surrounding air. Laser anemometry thus makes it possible to obtain a measurement of speed at a distance, outside of the flow disturbed by the rotor. In order to obtain the three components of the airspeed vector, at least three fixed laser beams are conventionally used, that is to say a beam scanning device making it possible to probe the atmosphere in the three dimensions. A rotating prism makes it possible for example to scan a cone having a small apex angle. However, in certain conditions of use of the helicopter, notably in the case of maneuvering close to obstacles (for example a building or a rock face), the presence of parasitic echoes capable of resulting in an unavailability or even a non-indicated degradation of the accuracy of the speed measurement can be feared in all or some of the sighting axes.
The document FR 2891409 relates to a laser anemometry device with improved eye safety using a scanning cone but not providing any processing capable of dealing with this risk of unavailability or even of loss of integrity of the airspeed measurement.